Hardwood to match the floors would be my first choice. What are the stairs now?

They had carpet. Removed it and stairs are not hardwood.

The stairs run down into a hallway and then more stairs to basement. So it would be expensive to do all that in hardwood.

Thinking something like this

Will black threads be difficult to see or is there adequate lighting?

It would be expensive to do it all in hardwood. I think you could probably do it with paint. I assume the existing treads are some other type of wood? You could do vinyl, but you'd want to add matching nosing and any change in step height could cause safety issues. Also, if you have a special step at the bottom (e.g., rounded) it would complicate things. It seems like too much work for what you will get. As long as the treads are in decent shape, I'd just fill any nail holes with wood filler and paint with a good floor/porch paint (probably need to prime first). Anything really dark, whether it's paint or vinyl will show more dust/lint, etc. Think about a nice stair runner in the middle. It will help disguise the fact that the treads are different than the floors.